Marshall has given no sign that he wouldn't be able to keep up his solid perfomance all year. Unless he has a few major blowups, he's not going anywhere in that rotation, regardless of when Miller comes back.
And there is no way Maddux gets dealt this year. If anything, he is soldifying a reason to bring him back next year. Having 3 young, promising pitchers in the dugout to pick his brain is worth a ton itself.